Stationary and dynamic radial transverse electric polarizer for high numerical aperture systems

ABSTRACT

A radial transverse electric polarizer device is provided. The device includes a first layer of material having a first refractive index, a second layer of material having a second refractive index, and a plurality of elongated elements azimuthally and periodically spaced apart, and disposed between the first layer and the second layer. The plurality of elongated elements interact with electromagnetic waves of radiation to transmit transverse electric polarization of electromagnetic waves of radiation. One aspect of the invention is, for example, to use such polarizer device in a lithographic projection apparatus to increase imaging resolution. Another aspect is to provide a device manufacturing method including polarizing a beam of radiation in a transverse electric polarization.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to optical polarizers in generaland more particularly to polarizers for high numerical aperturelithography.

[0003] 2. Background of the Invention

[0004] A lithographic projection apparatus can be used, for example, inthe manufacture of integrated circuits (ICs). In such a case, apatterning device generates a circuit pattern corresponding to anindividual layer of the IC, and this pattern can be imaged onto a targetportion (e.g. comprising one or more dies) on a substrate (siliconwafer) that has been coated with a layer of radiation sensitive material(resist). In general, a single wafer or substrate will contain a wholenetwork of adjacent target portions that are successively irradiated viathe projection system, one at a time.

[0005] The term “patterning device” as here employed should be broadlyinterpreted as referring to device that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate. Theterm “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device.

[0006] An example of such a patterning device is a mask. The concept ofa mask is well known in lithography, and it includes mask types such asbinary, alternating phase shift, and attenuated phase shift, as well asvarious hybrid mask types. Placement of such a mask in the radiationbeam causes selective transmission (in the case of a transmissive mask)or reflection (in the case of a reflective mask) of the radiationimpinging on the mask, according to the pattern on the mask. In the caseof a mask, the support structure will generally be a mask table, whichensures that the mask can be held at a desired position in the incomingradiation beam, and that it can be moved relative to the beam if sodesired.

[0007] Another example of a patterning device is a programmable mirrorarray. One example of such an array is a matrix-addressable surfacehaving a viscoelastic control layer and a reflective surface. The basicprinciple behind such an apparatus is that, for example, addressed areasof the reflective surface reflect incident light as diffracted light,whereas unaddressed areas reflect incident light as undiffracted light.Using an appropriate filter, the undiffracted light can be filtered outof the reflected beam, leaving only the diffracted light behind. In thismanner, the beam becomes patterned according to the addressing patternof the matrix addressable surface.

[0008] An alternative embodiment of a programmable mirror array employsa matrix arrangement of tiny mirrors, each of which can be individuallytilted about an axis by applying a suitable localized electric field, orby employing piezoelectric actuators. Once again, the mirrors are matrixaddressable, such that addressed mirrors will reflect an incomingradiation beam in a different direction to unaddressed mirrors. In thismanner, the reflected beam is patterned according to the addressingpattern of the matrix-addressable mirrors. The required matrixaddressing can be performed using suitable electronics. In both of thesituations described hereabove, the patterning device can comprise oneor more programmable mirror arrays. More information on mirror arrays ashere referred to can be seen, for example, from United States PatentsU.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597and WO 98/33096. In the case of a programmable mirror array, the supportstructure may be embodied as a frame or table, for example, which may befixed or movable as required.

[0009] Another example of a patterning device is a programmable LCDarray. An example of such a construction is given in U.S. Pat. No.5,229,872. As above, the support structure in this case may be embodiedas a frame or table, for example, which may be fixed or movable asrequired.

[0010] For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table. However, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

[0011] In current apparatus, employing patterning by a mask on a masktable, a distinction can be made between two different types of machine.In one type of lithographic projection apparatus, each target portion isirradiated by exposing the entire mask pattern onto the target portionat once. Such an apparatus is commonly referred to as a wafer stepper.In an alternative apparatus, commonly referred to as a step and scanapparatus, each target portion is irradiated by progressively scanningthe mask pattern under the projection beam in a given referencedirection (the “scanning” direction) while synchronously scanning thesubstrate table parallel or anti-parallel to this direction. Since, ingeneral, the projection system will have a magnification factor M(generally<1), the speed V at which the substrate table is scanned willbe a factor M times that at which the mask table is scanned. Moreinformation with regard to lithographic devices as here described can beseen, for example, from U.S. Pat. No. 6,046,792.

[0012] In a known manufacturing process using a lithographic projectionapparatus, a pattern (e.g. in a mask) is imaged onto a substrate that isat least partially covered by a layer of radiation sensitive material(resist). Prior to this imaging, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake and measurementand/or inspection of the imaged features. This array of procedures isused as a basis to pattern an individual layer of a device, e.g. an IC.Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation, chemical,mechanical polishing, etc., all intended to finish off an individuallayer. If several layers are required, then the whole procedure, or avariant thereof, will have to be repeated for each new layer and theoverlay (juxtaposition) of the various stacked layers is performed asaccurate as possible. For this purpose, a small reference mark isprovided at one or more positions on the wafer, thus defining the originof a coordinate system on the wafer. Using optical and electronicdevices in combination with the substrate holder positioning device(referred to hereinafter as “alignment system”), this mark can then berelocated each time a new layer has to be juxtaposed on an existinglayer, and can be used as an alignment reference. Eventually, an arrayof devices will be present on the substrate (wafer). These devices arethen separated from one another by a technique such as dicing or sawing,whence the individual devices can be mounted on a carrier, connected topins, etc. Further information regarding such processes can be obtained,for example, from the book “Microchip Fabrication: A Practical Guide toSemiconductor Processing”, Third Edition, by Peter van Zant, McGraw HillPublishing Co., 1997, ISBN 0-07-067250-4.

[0013] For the sake of simplicity, the projection system may hereinafterbe referred to as the “lens.” However, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens.”Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and WO98/40791.

[0014] Development of new tools and methods in lithography have lead toimprovements in resolution of the imaged features patterned on a device,e.g. an IC. Tools and techniques in optical lithography continue toimprove possibly leading to a resolution of less than 50 nm. This may beaccomplished using relatively high numerical aperture (NA) lenses(greater than 0.75 NA), wavelengths down to 157 nm, and a plethora oftechniques such as phase shift masks, non-conventional illumination andadvanced photoresist processes.

[0015] The success of manufacturing processes at these sub-wavelengthresolutions will rely on the ability to print low modulation images orthe ability to increase the image modulation to a level that will giveacceptable lithographic yield.

[0016] Typically, the industry has used the Rayleigh criterion toevaluate the resolution and depth of focus capability of a process. Theresolution and depth of focus (DOF) are the following equations:

Resolution=k ₁ (λ/NA),

[0017] and

DOF=k ₂(λ/NA²),

[0018] where λ is the wavelength of the illumination source and k₁ andk₂ are constants for a specific lithographic process.

[0019] Therefore, for a specific wavelength, as resolution is increasedthrough the use of higher-NA tools, the depth of focus can decrease. Theloss in DOF with high NA is well known. However, the polarizationtargets for high NA partially coherent systems have not been examined.According to the following equation:

I(r, Z ₀)=Σ_(i)∫_(s) dρJ(ρ₀) ≡FT {O(ρ-ρ₀)P _(i)(ρ)F _(i)(ρ,z)H(ρ,Z ₀)

[0020] where the image I, in a given film such as a photoresist, is afunction of position r and specific for a given focus position Z₀. Thisequation is valid for all NAs and the image is the summation over allpolarization states i. The integral is over the source distributiondefined by J. The Fourier term within brackets represents the electricfield distribution at the exit pupil. The 4 terms inside the bracketare, respectively, the object spectrum O of the reticle pattern, apolarization function P, a film function F and a pupil function H.

[0021] According to this equation, high NA imaging is intrinsicallylinked with the polarization state and the thin film structure, wherethe electric field coupling and the power absorbed by a photoresist filmcan be drastically altered. The power absorbed due to incident planewaves on a photoresist film are proportional to the exposure necessaryto develop the film.

[0022] Studies by Donis G. Flagello et al. published under the title“Optical Lithography into the Millennium: Sensitivity to Aberrations,Vibrations and Polarization,” in the 25th Annual International Symposiumon Microlithography, SPIE, Feb. 27-Mar. 3, 2002, Santa Clara, Calif.,USA, have shown that two orthogonal polarization (Transverse Electricpolarization TE and Transverse Magnetic Polarization TM) divergeextensively at high NA, up to a 25% power change. An imaging systemwould contain a multitude of incident angles, reducing this effect.However, alternating phase shift masks (PSMs) require a small partialcoherence which restricts the total number of angles and thus couldproduce similar exposure changes.

[0023] Results have been obtained through simulation which show that acritical dimension difference from a completely polarized state and theunpolarized state depends on the numerical aperture NA. Results havealso shown that dense lines with an alternating phase shift mask (PSM)is the most critical feature and this has been explained by the factthat the pupil configuration essentially produces 2-beam interference atthe wafer level and this case tends to maximize the effects ofpolarization. If, for example, a numerical aperture of 0.85 (relativelyhigh) is selected and one wanted to limit the systematic criticaldimension CD error to less than 3%, the residual polarization should belimited to 10%. The critical dimension CD is the smallest width of aline or the smallest space between two lines permitted in thefabrication of a device. The simulation results also indicate the levelof pupil filling and partial coherence can lessen the effects ofpolarization. This has been shown by the small polarization influence onthe features using conventional illumination.

[0024] Therefore, as more phase masks are used and imaging technologythat demands small coherence levels is used, newer metrologytechnologies for the lens may be required. For example, high NApolarization effects may result in extremely tight specifications onillumination polarization for lithography tools.

[0025] The advent of a resolution-enhancement technique (RET) called“liquid immersion” promises extending 157 nm optical lithography to wellbelow 70 nm and possibly below 50 nm without changes in illuminationsources (lasers) or mask technology. According to a MassachusettsInstitute of Technology (MIT) article by M. Switkes et al. entitled“Immersion Lithography at 157 nm” published in J. Vac. Sci. Technology B19(6), Nov/Dec 2001, liquid immersion technology could potentially pushout the need for next-generation lithography (NGL) technologies such asextreme ultraviolet (EUV) and electron projection lithography (EPL). Theliquid immersion technology involves using chemicals and resists toboost resolution. Immersion lithography can enhance the resolution ofprojection optical systems with numerical apertures up to the refractiveindex of the immersion fluid. The numerical aperture NA is equal to theproduct of the index n of the medium and the sinus of the half angle θof the cone of light converging to a point image at the wafer (NA=n sinθ). Thus, if NA is increased by increasing the index n, the resolutioncan be increased (see equation: Resolution=k₁ (λ/NA)). However, asstated above, higher NA may result in extremely tight specifications onillumination polarization for lithography tools. Therefore, polarizationplays an increased role in immersion lithography.

SUMMARY OF THE INVENTION

[0026] It is an aspect of the present invention to provide a radialtransverse electric polarizer device including a first layer of materialhaving a first refractive index, a second layer of material having asecond refractive index, and a plurality of elongated elementsazimuthally and periodically spaced apart, and disposed between thefirst layer and the second layer. The plurality of elongated elementsinteract with electromagnetic waves of radiation to transmit transverseelectric polarization of electromagnetic waves of radiation.

[0027] In one embodiment, the first refractive index is equal to thesecond refractive index. In another embodiment the plurality ofelongated elements form a plurality of gaps. These gaps may include, forexample, air or a material having a third refractive index. In yetanother embodiment, the elongated elements periodically are spaced apartwith a period selected to polarize the electromagnetic waves ofradiation in a transverse electric polarization. In one embodiment theelectromagnetic radiation is ultraviolet radiation.

[0028] It is another aspect of the present invention to provide a radialtransverse electric polarizer device including a substrate materialhaving a first refractive index, and a plurality of elongatedazimuthally oriented elements coupled to said substrate material and theelongated elements having a second refractive index. The plurality ofelements are periodically spaced apart to form a plurality of gaps suchthat the radial transverse electric polarizer device interacts with anelectromagnetic radiation having first and second polarizations toreflect substantially all of the radiation of the first polarization andtransmit substantially all of the radiation of the second polarization.

[0029] In an embodiment of the present invention the first polarizationis a transverse magnetic polarization (TM) and the second polarizationis a transverse electric (TE) polarization. The plurality of elongatedelements can be formed of, for example, aluminum, chrome, silver andgold. The substrate material can be, for example, quartz, silicon oxide,silicon nitride, gallium arsenide a dielectric material, or combinationsthereof.

[0030] According to another aspect of the invention a lithographicprojection apparatus is provided, the apparatus including a radiationsystem constructed and arranged to provide a projection beam ofradiation, a support structure constructed and arranged to supporting apatterning device, the patterning device constructed and arranged topattern the projection beam according to a desired pattern, a substratetable to hold a substrate, a projection system constructed and arrangedto project the patterned beam onto a target portion of the substrate,and a polarizer device constructed and arranged to polarize the beam ofradiation in a transverse electric polarization direction.

[0031] A further aspect of the invention there is provided a devicemanufacturing method including providing a substrate that is at leastpartially covered by a layer of radiation-sensitive material, providinga projection beam of radiation using a radiation system, using apatterning device to endow the projection beam with a pattern in itscross-section, projecting the patterned beam of radiation onto a targetportion of the layer of radiation-sensitive material; and polarizingsaid beam of radiation in a transverse electric polarization. Stillanother aspect of the invention is to provide a device manufactured adevice using the above method.

[0032] Although specific reference may be made in this text to the useof the apparatus according to the invention in the manufacture of ICs,it should be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid crystal display panels,thin film magnetic heads, etc. One will appreciate that, in the contextof such alternative applications, any use of the terms “reticle”,“wafer” or “die” in this text should be considered as being replaced bythe more general terms “mask”, “substrate” and “target portion”,respectively.

[0033] In the present document, the terms “radiation” and “beam” areused to encompass all types of electromagnetic radiation, includingultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or126 nm) and EUV (extreme ultra-violet radiation, e.g. having awavelength in the range 5-20 nm), as well as particle beams, such as ionbeams or electron beams.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] These and other objects and features of the invention will becomemore apparent and more readily appreciated from the following detaileddescription of the presently preferred exemplary embodiments of theinvention, taken in conjunction with the accompanying drawings, ofwhich:

[0035]FIG. 1 schematically depicts a lithographic projection apparatusaccording to an embodiment of the invention;

[0036]FIG. 2A is a schematic illustration of a radial polarizeraccording to an embodiment of the present invention;

[0037]FIG. 2B is an enlarged view of gratings at an area of polarizerdepicted in FIG. 2A;

[0038]FIG. 3 is an enlarged lateral view of the radial polarizeraccording to another embodiment of the present invention;

[0039]FIG. 4 is a vector diagram showing the preferential polarizationdirection and the output from a TE polarizer according to theembodiments shown in FIGS. 2A and 3;

[0040]FIG. 5 is a plot of the exposure latitude versus depth of focusfor a comparative example 1;

[0041]FIG. 6 is a plot of the exposure latitude versus depth of focusfor an example 1 of the present invention;

[0042]FIG. 7 is a schematic illustration of a radial polarizer accordingto an alternative embodiment of the present invention;

[0043]FIG. 8 shows schematically an embodiment of a lithographic systemutilizing the radial TE polarizer of the present invention;

[0044]FIG. 9 is flow-chart representing a device manufacturing methodaccording to the present invention; and

[0045]FIG. 10 is a schematic illustration of another embodiment of apolarizer according to the present invention.

DETAILED DESCRIPTION

[0046] Several techniques have been used to create polarized light.There are basically four techniques for polarizing a natural beam oflight, i.e. non-polarized light. One technique is based on birefringentor biaxial materials. A second technique is based on the use of dichroicmaterials such as “polaroid.” A third technique employs thin-filmtechnology and it uses Brewster's effect. A fourth technique is based onwire grids or conductive gratings.

[0047] The use of birefringent materials to polarize light is known inthe production of birefringent polarizers. Birefringent polarizers canbe made from many crystals and also certain stretched polymers.Birefringent materials are materials having a different optical index inone direction compared to another. The degree of difference in theoptical index between the two directions varies with the wavelength ofthe light. The difference in index is used to separate beams of onelinear polarization from another. Use of birefringent polarizers ischaracterized by inefficiency, wavelength dependent performance andrequires highly collimated light. For these reasons birefringentpolarizers are not commonly used in optical projection systems.

[0048] Dichroic polarizers are polarizers designed to absorb onepolarization and transmit the other one. Most commonly used dichroicpolarizers consist of a polymeric sheet stretched to orient itsmolecules and treated with iodine and/or other materials or chemicalssuch that the molecules absorb polarization of one orientation. Strechedpolymers polarizers absorb all the intensity of one polarization and atleast 15% of the transmitted polarization. Stretched polymer polarizersdegrade with time as the light induces chemicals changes in thepolymeric material resulting in the material becoming yellow or brittle.Dichroic polarizers are also sensitive to heat and other environmentalchanges.

[0049] In the last decade a polarizer device has been developed in whichstretched polymer sheets are made birefringent. These stretched sheetsreflect one polarization and pass the other. One problem with thispolarizer technique is its low extinction ratio of approximately 15.While useful for some applications, this extinction ratio is notadequate for imaging applications without a secondary polarizer. Thistype of polarizer also suffers from the environmental problems discussedabove.

[0050] Thin film polarizer technology uses Brewster's effect in which alight beam incident on a surface of a material such as glass, plastic orthe like, at Brewster's angle (near 45 degrees) is divided into twopolarized beams one transmitted and the other one reflected. Thin filmpolarizer technology however limits the angular range of the light beamincidence. The acceptance angular range is very narrowly limited to afew degrees in most devices. Thin film polarizer technology also suffersfrom the wavelength dependence because of the dependence of Brewster'sangle on the wavelength of the incident light.

[0051] For an image projection system where applications of a polarizedlight beam are sought, a brighter beam is always desirable. Thebrightness of a polarized beam is determined by numerous factors, one ofthe factors being the light source itself. Another factor for a systemthat employs a polarizer is the angle of acceptance. A polarizer with anarrow or limited acceptance angle cannot gather as much light from adivergent source as a system that employs a wide acceptance angle. Apolarizer with large acceptance angles allows flexibility in the designof a projection optical system. This is because it is not necessary forthe polarizer to be positioned and oriented within a narrow range ofacceptance angles with respect to the light source.

[0052] Another desired characteristic for a polarizer is to effectivelyseparate one component of polarization from the other component. This iscalled the extinction ratio, which is the ratio of the amount of lightof the desired polarization component to the amount of light of theundesired polarization component. Other desired characteristics includefreedom of positioning the polarizer in an optical projection systemwithout diminishing the efficiency of the polarizer and/or introducingadditional restrictions on the system such as orientation of the beametc.

[0053] Another polarization technique utilizes a conductive grating orwire grid. A wire grid polarizer is a planar assembly of evenly spacedparallel electrical conductors whose length is much larger than theirwidth and the spacing between the conductive elements is less than thewavelength of the highest frequency light component of the incidentlight beam. This technique has been successfully used for a number ofyears in the radio frequency domain and up to the infrared region of thespectrum. Waves with a polarization parallel to the conductors (Spolarization) are reflected while waves of orthogonal polarization (Ppolarization) are transmitted through the grid. The wire grid polarizeris used mainly in the field of radar, microwaves, and infrared.

[0054] The wire grid polarizer technique has not been used for shorterwavelengths except in few instances in the visible wavelengths range.For example, in U.S. Pat. No. 6,288,840 a wire grid polarizer for thevisible spectrum is disclosed. The wire grid polarizer is imbedded in amaterial such as glass and includes an array of parallel elongatedspaced-apart elements sandwiched between first and second layers of thematerial. The elongated elements form a plurality of gaps between theelements which provide a refractive index less than the refractive indexof the first layer. The array of elements is configured to interact withelectromagnetic waves in the visible spectrum to reflect most of thelight of a first polarization and transmit most of the light of a secondpolarization. The elements have a period less than 0.3 microns andwidths less than 0.15 microns.

[0055] Another instance where a wire grid polarizer is used forpolarization in the visible spectrum is described in U.S. Pat. No.5,383,053. A wire grid polarizer is used in a virtual image display toimprove reflection and transmission efficiency over conventional beamsplitters. The wire grid polarizer is used as a beam splitting elementin an on-axis, polarized virtual image display. The extinction ratio ofthe grid polarizer was not an issue in this application because theimage was already polarized and only the relatively high efficiency ofthe reflection and transmission was of interest in this application.

[0056] Lopez et al., in an article published in Optics Letters, Vol. 23,No. 20, pp. 1627-1629, describe the use of surface-relief gratingpolarization, similar to wire grid technology. Lopez et al. describe theuse of grating polarization in the visible spectrum (output of a He-Nelaser at 632.8 nm) as a quarter-wave plate polarizer (phase retardance,π/2) at normal incidence and as a polarizing beam splitter (PBS) at anangle of incidence of 40°. The polarizer is a one-dimensionalsurface-relief grating with a period of 0.3 microns and a 50% dutycycle. The grating material is a single layer of SiO₂ (index ofrefraction 1.457) sandwiched between two layers of Si₃N₄ (index ofrefraction 2.20) upon a fused-quartz substrate.

[0057] The wire grid polarizer technology has not been, however,suggested for use in the ultraviolet wavelengths range, i.e. shorterthan the visible lower limit wavelength of 400 nm. As stated above,development of a polarizer for ultraviolet radiation would allowincreases in resolution of lithographic projection systems, and morespecifically increases in the resolution of lithographic projectionsystems having high NA, such as in the case of immersion lithographicsystems.

[0058] Ferstl et al., in an article published in SPIE Vol. 3879, Sept.1999, pp. 138-146, discloses the use of “high-frequency” gratings aspolarization elements. Binary gratings with feature sizes smaller thanthe illumination wavelength of 650 nm were fabricated in quartz glass bymicrostructuring techniques using direct electron-beam writing combinedwith successive reactive ion etching. In polarization beam splittersdiffraction efficiencies of about 80% in the −1 order for transverseelectric TE polarization and 90% in the 0 order for transverse magneticTM polarization were obtained.

[0059] The polarization state of a wave can be defined by two parametersθ and φ, where θ defines the relative magnitudes of TE and TM wavecomponents, and φ defines their relative phase. The incident wave can beexpressed by the following pair of equations:

A_(TE)=cos θ and A_(TM)=e^(jφ) sin θ

[0060] Thus, for φ=0, the wave is linearly polarized at an angle θ.Circular polarization is obtained when θ=π/4 and φ=±π/2. A TE polarizedwave is represented by θ=0. A TM wave is represented by θ=π/2. TE and TMpolarizations are fundamental polarization components.

[0061] Before going into details about polarization systems andpolarization lenses it would be judicious to put the polarization in thecontext of its application, i.e. in the context of lithographic toolsand methods.

[0062]FIG. 1 schematically depicts a lithographic projection apparatus 1according to an embodiment of the invention. The apparatus 1 includes aradiation system Ex, IL constructed and arranged to supply a projectionbeam PB of radiation (e.g. EUV radiation), which in this particular casealso comprises a radiation source LA; a first object table (mask table)MT provided with a mask holder that holds a mask MA (e.g. a reticle),and connected to a first positioning device PM that accurately positionsthe mask with respect to a projection system PL. A second object table(substrate table) WT provided with a substrate holder that holds asubstrate W (e.g. a resist-coated silicon wafer), and connected to asecond positioning device PW that accurately positions the substratewith respect to the projection system PL. The projection system (“lens”)PL (e.g. a mirror group) is constructed and arranged to image anirradiated portion of the mask MA onto a target portion C (e.g.comprising one or more dies) of the substrate W.

[0063] As here depicted, the apparatus is of a transmissive type (i.e.has a transmission mask). However, in general, it may also be of areflective type, for example (with a reflective mask). Alternatively,the apparatus may employ another kind of patterning device, such as aprogrammable mirror array of a type as referred to above.

[0064] The source LA (e.g. a discharge or laser-produced plasma source)produces a beam of radiation. This beam is fed into an illuminationsystem (illuminator) IL, either directly or after having traversed aconditioning device, such as a beam expander Ex, for example. Theilluminator IL may comprise an adjusting device AM that sets the outerand/or inner radial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in the beam. In addition, itwill generally comprise various other components, such as an integratorIN and a condenser CO. In this way, the beam PB impinging on the mask MAhas a desired uniformity and intensity distribution in itscross-section.

[0065] It should be noted with regard to FIG. 1 that the source LA maybe within the housing of the lithographic projection apparatus (as isoften the case when the source LA is a mercury lamp, for example), butthat it may also be remote from the lithographic projection apparatus,the radiation beam which it produces being led into the apparatus (e.g.with the aid of suitable directing mirrors). This latter scenario isoften the case when the source LA is an excimer laser. The presentinvention encompasses both of these scenarios.

[0066] The beam PB subsequently intercepts the mask MA, which is held ona mask table MT. Having traversed the mask MA, the beam PB passesthrough the lens PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the second positioning device PW andinterferometer IF, the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of the beamPB. Similarly, the first positioning device PM can be used to accuratelyposition the mask MA with respect to the path of the beam PB, e.g. aftermechanical retrieval of the mask MA from a mask library, or during ascan. In general, movement of the object tables MT, WT will be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which are not explicitlydepicted in FIG. 1. However, in the case of a wafer stepper (as opposedto a step and scan apparatus) the mask table MT may just be connected toa short stroke actuator, or may be fixed. The mask MA and the substrateW may be aligned using mask alignment marks M₁, M₂ and substratealignment marks P₁, P₂.

[0067] The depicted apparatus can be used in two different modes. Instep mode, the mask table MT is kept essentially stationary, and anentire mask image is projected at once, i.e. a single “flash,” onto atarget portion C. The substrate table WT is then shifted in the X and/orY directions so that a different target portion C can be irradiated bythe beam PB.

[0068] In scan mode, essentially the same scenario applies, except thata given target portion C is not exposed in a single “flash.” Instead,the mask table MT is movable in a given direction (the so-called “scandirection”, e.g., the Y direction) with a speed v, so that theprojection beam PB is caused to scan over a mask image. Concurrently,the substrate table WT is simultaneously moved in the same or oppositedirection at a speed V=Mv, in which M is the magnification of the tensPL (typically, M=¼ or ⅕). In this manner, a relatively large targetportion C can be exposed, without having to compromise on resolution.

[0069] Currently, lenses that are used in projection lithography do notuse TE polarizers. They either have linear polarization or circularpolarization. The polarization state in the lithography tools used priorto the present invention are either linear, circular or unpolarized. Theinventors have determined that in order to improve resolution and allowbetter imaging under high NA, such as in immersion lithography where NAis greater than 1, it will require suppression of TM polarization forall feature orientations. Otherwise the loss of contrast would be severeenough to destroy any viable imaging.

[0070] In order to eliminate TM polarization and only use TEpolarization in lithographic projection, the inventors have found thatusing radial polarizers in circularly symmetric lenses allows forselective elimination of the TM polarization component. The manufactureof radial polarizers is similar to that of wire grid technologydescribed previously. It is accomplished by the manufacture of radialperiodic metal lines such as, for example chrome or silver, dielectricsor multilayers, either on a lens element or embedded within the lenselement.

[0071]FIG. 2A is a schematic illustration of an embodiment of a radialpolarizer according to the present invention. Radial polarizer 20 hasperiod gratings 22 arranged in a radially symmetric pattern. The periodof the grating can be selected for a specific wavelength of radiationused and in accordance with other desired parameters. In thisembodiment, the gratings are deposited on a substrate 24, which can beglass or other materials. The gratings 22 can be, for example, a metalsuch as aluminum, chrome, silver, gold or any material that isconductive at the wavelength the electromagnetic radiation beam. Thegratings can also be made, for example, of dielectrics or a combinationin a multilayer structure such as, but not limited to, a single layer ofSiO₂ sandwiched between two layers of Si₃N₄ on a fused-quartz substrate.The gratings 22 may also be etched using electron beams, for example,following a pattern transferred to a substrate of GaAs.

[0072]FIG. 2B is an enlarged view of gratings 22 at area 26 of polarizer20. As shown in FIG. 2B gratings 22 are interlaced to allow smoothtransitions of the polarization effects to maintain uniformity of the TEpolarization intensity along the diameter of the polarizer.

[0073] Although the polarizer 22 is illustrated in FIG. 2A having a diskshape, the polarizer 20 can also be of a polygonal shape such as, butnot limited to, a rectangular shape, hexagonal shape, etc.

[0074]FIG. 3 is an enlarged lateral view of another embodiment of theradial polarizer. Radial polarizer 30 includes a first layer of material32 having a first refractive index, a second layer of material 34 havinga second refractive index. A plurality of elongated elements 36 (orgratings) azimuthally and periodically spaced apart are disposed betweenthe first layer 32 and the second layer 34. The plurality of elongatedelements 36 interact with electromagnetic waves of light or radiation totransmit transverse electric TE polarization and reflect or absorb TMpolarization. The plurality of elongated elements 36 can be made, forexample, of silicon dioxide and the first and/or second layers 32 and/or34 can be made of any material comprising, for example, quartz,silicone, dioxide, silicon nitride, gallium arsenide etc. or adielectric material at the wavelength of the electromagnetic beam ofradiation. Similarly to the previous embodiment the spacing or periodbetween the elongated elements 36 can be selected according to theintended use of the polarizer, i.e. for a specific wavelength and inaccordance with other parameters in the lithographic system.

[0075] Similarly, although the polarizer 30 is illustrated in FIG. 3having a disk shape, the polarizer 30 can also be of a polygonal shapesuch as, but not limited to, a rectangular shape, hexagonal shape, etc.

[0076] Light impinging on polarizer 20, 30 at near normal incidencewould have its polarization state altered such that the output oftransmitted polarization state is orthogonal to the direction of thegrating lines 22, 36 in the polarizer 20, 30.

[0077]FIG. 4 is a vector diagram 40 with the preferential polarizationdirection 41 and output from TE polarizer 20. Higher errors and defectswould be allowed towards the center of the polarizer as the need for TEpolarization with high NA systems is greater at the edge of the pupil. Acoherent light illuminating through dense lines (reticle image lines)will produce 3 orders of diffraction. At 42 would be the position of the0 order diffraction of the beam of light and at 44 and 45, therespective positions of the +1 order diffraction and −1 orderdiffraction, for a vertical line. At 46 and 47, the respective positionsof +1 order diffraction and -1 order of diffraction, for a horizontalline. The +1 and −1 order will interfere giving rise to valleys andpeaks in the illumination reaching the wafer. If a TE polarization isused, for both vertical and horizontal lines, an interference patternoccurs leading to a high contrast and thus to a good resolution of thelines.

[0078] Whereas, in the case of linear polarization only one of thevertical or horizontal lines would lead to a clear interference patternwith high contrast. The other vertical or horizontal line would not becorrectly polarized, not form an interference pattern and thus thecontrast would be less. The combination of high and low contrast imageswould average out the result leading to a low definition or resolutionimaging for the overall pattern. To get rid of the component leading toabsence or minor interference at the wafer the inventors used a radialTE polarizer that allows interference patterns to occur in any azimuthaldirection in the lens. This would not be the case with circularpolarization as each component is a combination of two linear orthogonalpolarizations but can be thought of as turning in space but in a fixedmanner as function of position. Therefore, the use of circularpolarization would not lead to interference lines and consequently isnot suitable for high resolution imaging for lithographic systemsbecause in the wafer plane circular polarization is reduced to linearpolarization and the drawbacks of this were described above in thisparagraph.

[0079] In an immersion lithographic system, i.e. a lithographic systemwith a high NA, the use of a TE polarizer may be required in order toobtain the resolution adequate for imaging dense lines. FIG. 5 shows theprocess window for a comparative Example 1 unpolarized immersionlithographic system imaging 50 nm dense lines. The wavelength of use inthis example is 193 nm. The immersion fluid used is water with arefractive index of 1.437 (NA=1.437). The air equivalent numericalaperture NA is 1.29. The resist used in this example is Par710 on amatched substrate. The illumination is annular with σ=0.9/0.7. FIG. 5 isa plot of the exposure latitude versus depth of focus for comparativeExample 1. This plot indicates that the exposure latitude at a depth offocus of 0.0 is approximately 5.6%, which is an unusable level. At otherdepth of focus the exposure latitude decreases even more which makes anunpolarized light unusable in lithographic system at high NA.

[0080]FIG. 6 shows the process window for a 50 nm dense lines with TEpolarized light and immersion optics according to an Example 1 of thepresent invention. The wavelength of use in this example is 193 nm. Theimmersion fluid used is water with a refractive index of 1.437(NA=1.437). The resist used, in this example, is Par710 on a matchedsubstrate. The illumination is annular with σ=0.9/0.7. FIG. 6 is a plotof the exposure latitude amount versus depth of focus. This plotindicates that the exposure latitude at a depth of focus of 0.0 isapproximately 9.9%, which is a usable level. An improvement in exposurelatitude of 75% is obtained when using TE radial polarization system ofExample 1 of the present invention compared to comparative Example 1. Animprovement in DOF of 27% is obtained in Example 1 of the presentinvention compared to comparative Example 1. Thus an increased processedwindow is enabled by the use of the TE polarizer of the presentinvention. At other depth of focus the exposure latitude decreases withthe increase of the depth of focus.

[0081]FIG. 7 is a schematic illustration of another embodiment of aradial polarizer according to the present invention. Radial TE polarizer70 is comprised of a plurality of plate polarizers. Radial polarizer 70is fabricated by cutting the plate polarizer 72 that have linearpolarization preference. The plate polarizers are cut into plate sectors72 a-h in order to fabricate a circular-shaped piece polarizer. Theplate sectors 72 a-h are then assembled to form a radial polarizer 70.Each plate sector 72 a-h has a linear polarization vector state 74 a-hand thus by assembling the plate sectors 72 a-h in this fashion thelinear vector polarization 74 a-h would rotate to form radialpolarization configuration. However, since the plate sectors arediscrete elements, in order to obtain a “continuous” TE radialpolarization, polarizer 70 is preferably rotated to randomize opticalpath differences between the plates and to insure uniformity. Therotation of the polarizer is not necessary but in some cases it wouldadd uniformity and depending on how the rotation is implemented, thespeed of the rotation could be selected to be very slow or very fast. Toperform such rotation the polarizer 70 can be mounted for example on airbearings. In the case of EUV lithography where at least parts of thelithographic system are in vacuum, an alternative mount solution can beprovided. For example, the polarizer 70 can be mounted on magneticbearing systems instead of air bearings. The speed of rotation wouldgovern the uniformity of the polarization. In general, the rotation rateshould be sufficiently high to randomize optical path differencesbetween the plates in order to insure uniformity.

[0082]FIG. 8 shows schematically an embodiment of a lithographic systemutilizing a radial TE polarizer of the present invention. As describedpreviously, lithographic system 80 comprises illumination or radiationsystem source 81, mask or reticle 82, projection lens 83, substrate orwafer 84 and a radial TE polarizer 20, 30, or 70. The radial TEpolarizer 20, 30, or 70 is shown in this embodiment positioned at theentrance of the projection lens, optimally close to the pupil plane,however, one ordinary skill in the art would appreciate that the radialpolarizer 20, 30, or 70 can be positioned anywhere in the projectionlens or outside the projection lens such as, for example, between thereticle or mask 82 and the projection lens 83.

[0083] Referring to FIG. 9, a device manufacturing method according tothe present invention includes providing a substrate that is at leastpartially covered by a layer of radiation-sensitive material S110,providing a projection beam of radiation using a radiation system S120,using a patterning device to endow the projection beam with a pattern inits cross-section S130, projecting the patterned beam of radiation ontoa target portion of the layer of radiation-sensitive material S140, andpolarizing the beam of radiation in a transverse electric polarizationS150.

[0084]FIG. 10 is a schematic illustration of another embodiment of apolarizer 100 according to the present invention used to createtangential polarization. Conventional polarization systems are known touse polarization units such as beam-splitting cubes. Beam-splittingcubes consist of a pair of fused silica precision right-angle prismscarefully cemented together to minimize wave front distortion. Thehypotenuse of one of the prisms is coated with a multilayer polarizingbeam-splitter coating (such as a birefringent material) optimized for aspecific wavelength. The beam-splitter throws away an amount of incidentlight, and at the exit from the cube, in one of the two branches, thelight is linearly polarized. Conventionally, in order to preventdifferences in printing horizontal and vertical lines, the polarizationis rendered circular with a quarter wave plate, in the pupil of theimaging system.

[0085] However as stated previously, circular polarization is comprisedof both fundamental polarization components TE and TM. In accordancewith the present invention, a polarizer plate 102 is introduced in thepupil of the imaging system comprising the cube beam-splitter 103. Inone embodiment, the plate polarizer 102 comprises two half-wave plates104A and 104B. The plate polarizer 102 polarizes the linear polarizedlight into a first s-polarized light S1 and a second s-polarized lightS2 such that a wave vector S1 of the first s-polarized light and a wavevector S2 of the second polarized light are perpendicular to each other.The plate polarizer is disposed at the end of the cube-beam-splitter 103such that one polarization direction is limited to only two quarters ofthe pupil. This is suitable for printing horizontal lines since thepolarization arrives as s-polarization on the wafer. In the other twoquarters segments, a half-wave phase shift is introduced throughbirefringence (under 45 degrees). The polarization that was sagital willrotate over 90 degrees and becomes also tangential. This, in turn, issuitable for printing vertical lines. In other words, the firsts-polarized light S1 is used to print lines on a wafer in a horizontaldirection and the second s-polarized light S2 is used to print lines ona wafer in a vertical direction. In this way, S-polarization or TEpolarization is obtained for both vertical and horizontal lines.

[0086] Furthermore, since numerous modifications and changes willreadily occur to those of skill in the art, it is not desired to limitthe invention to the exact construction and operation described herein.Moreover, the process, method and apparatus of the present invention,like related apparatus and processes used in the lithographic arts tendto be complex in nature and are often best practiced by empiricallydetermining the appropriate values of the operating parameters or byconducting computer simulations to arrive at a best design for a givenapplication. Accordingly, all suitable modifications and equivalentsshould be considered as falling within the spirit and scope of theinvention.

What is claimed is:
 1. A radial transverse electric polarizer device,comprising: a first layer of material having a first refractive index; asecond layer of material having a second refractive index; and aplurality of elongated elements azimuthally and periodically spacedapart, and disposed between said first layer and said second layer,wherein said plurality of elongated elements interact withelectromagnetic waves of radiation to transmit transverse electricpolarization of electromagnetic waves of radiation.
 3. A radialtransverse electric polarizer device according to claim 1, wherein saidfirst refractive index is equal to said second refractive index.
 4. Aradial transverse electric polarizer device according to claim 1,wherein said plurality of elongated elements form a plurality of gaps.5. A radial transverse electric polarizer device according to claim 4,wherein said gaps include air.
 6. A radial transverse electric polarizerdevice according to claim 4, wherein said gaps include a material havinga third refractive index.
 7. A radial transverse electric polarizerdevice according to claim 1, wherein said elongated elements have afourth refractive index.
 8. A radial transverse electric polarizerdevice according to claim 1, wherein said elongated elementsperiodically are spaced apart with a period selected to polarize saidelectromagnetic waves of light in a transverse electric polarization. 9.A radial transverse electric polarizer device according to claim 1,wherein said electromagnetic radiation is ultraviolet radiation.
 10. Aradial transverse electric polarizer device, comprising: a substratematerial having a first refractive index; and a plurality of elongatedazimuthally oriented elements coupled to said substrate material, saidelongated elements having a second refractive index, wherein saidplurality of elements are periodically spaced apart to form a pluralityof gaps such that said radial transverse electric polarizer deviceinteracts with an electromagnetic radiation comprising first and secondpolarizations to reflect substantially all of the radiation of the firstpolarization and transmit substantially all of the radiation of thesecond polarization.
 11. A radial transverse electric polarizer deviceaccording to claim 10, wherein said first polarization is a transversemagnetic polarization and said second polarization is a transverseelectric polarization.
 12. A radial transverse electric polarizer deviceaccording to claim 10, wherein said plurality of elongated elements areformed of an electrically conductive material at a wavelength of saidelectromagnetic radiation.
 13. A radial transverse electric polarizerdevice according to claim 12, wherein said electrically conductivematerial is selected from the group consisting of aluminum, chrome,silver and gold.
 14. A radial transverse electric polarizer deviceaccording to claim 10, wherein said substrate material is formed of adielectric material at a wavelength of said electromagnetic radiation.15. A radial transverse electric polarizer device according to claim 14,wherein said dielectric material is selected from the group consistingof quartz, silicon oxide, silicon nitride, gallium arsenide orcombinations thereof.
 16. A radial transverse electric polarizer deviceaccording to claim 10, wherein said substrate material comprises adielectric material.
 17. A lithographic projection apparatus,comprising: a radiation system constructed and arranged to provide aprojection beam of radiation; a support structure constructed andarranged to supporting a patterning device, the patterning deviceconstructed and arranged to pattern the projection beam according to adesired pattern; a substrate table to hold a substrate; a projectionsystem constructed and arranged to project the patterned beam onto atarget portion of the substrate; and a polarizer device constructed andarranged to polarize said beam of a radiation in a transverse electricpolarization direction.
 18. A lithographic projection apparatusaccording to claim 17, wherein said polarizer device comprises: a firstlayer of material having a first refractive index; a second layer ofmaterial having a second refractive index; and a plurality of elongatedelements azimuthally and periodically spaced apart, and disposed betweensaid first layer and said-second layer, wherein said plurality ofelongated elements interact with said beam of radiation to transmittransverse electric polarization of said beam of radiation.
 19. Alithographic projection apparatus according to claim 17, wherein saidpolarizer device comprises: a substrate material having a firstrefractive index; a plurality of elongated azimuthally oriented elementscoupled to said substrate material, said elongated elements having asecond refractive index, and wherein said plurality of elements areperiodically spaced apart to form a plurality of gaps such that saidradial transverse electric polarizer device interacts with the beam ofradiation comprising first and second polarizations to reflectsubstantially all of the radiation of the first polarization andtransmit substantially all of the radiation of the second polarization.20. A lithographic projection apparatus according to claim 17, wherein awavelength range of said radiation beam is in the ultraviolet spectrum.21. A lithographic projection apparatus according to claim 20, whereinsaid wavelength range is between 365 nm and 126 nm.
 22. A lithographicprojection apparatus according to claim 20, wherein said wavelengthrange is in the extreme ultraviolet.
 23. A radial transverse electricpolarizer device which interacts with an electromagnetic radiationcomprising first and second polarizations to reflect substantially allof the radiation of the first polarization and transmit substantiallyall of the radiation of the second polarization, said polarizer devicecomprising: a plurality of sector-shaped linear polarizer plates, eachdefining a plurality of parallel linear polarization orientations,wherein said plurality of sector-shaped linear polarizer plates areazimuthally arranged such that said plurality of parallel linearpolarization orientations rotate to form a radial polarizationconfiguration.
 24. A radial transverse electric polarizer deviceaccording to claim 23, wherein said radial transverse polarizer isconstructed and arranged to rotate around an axis perpendicular to aplane defined by said radial transverse polarizer.
 25. A devicemanufacturing method, comprising: providing a substrate that is at leastpartially covered by a layer of radiation-sensitive material; providinga projection beam of radiation using a radiation system; using apatterning device to endow the projection beam with a pattern in itscross-section; projecting the patterned beam of radiation onto a targetportion of the layer of radiation-sensitive material; and polarizingsaid beam of radiation in a transverse electric polarization.
 26. Adevice manufactured according to the method of claim
 25. 27. Atangential polarizer device, comprising: a cube beam-splitter polarizerconstructed and arranged to polarize at least a portion of an incidentlight into a linear polarized light; and a polarizing plate comprisingtwo half-wave plates, wherein said polarizing plate is disposed at anend of said cube beam-splitter polarizer to polarize said linearpolarized light into a first s-polarized light and a second s-polarizedlight such that a wave vector of said first s-polarized light and a wavevector of said second s-polarized light are perpendicular to each other.28. The tangential polarizer device according to claim 27, wherein saidfirst s-polarized light is used to print lines on a wafer in ahorizontal direction and said second s-polarized light is used to printlines on a wafer in a vertical direction.